This invention relates to the application of a thin layer of a liquid film to the surface of a substrate or wafer. More specifically, this invention relates to the deposition of a thin liquid film of the surface of a wafer with topography previously formed on the surface of the wafer.
Thin films of material are often applied to the surfaces of semiconductor wafers for various purposes. Photoresist may be spun on, using centripetal force to move the liquid resist across the wafer to form a thin film. Adhesives may also be applied, which may allow the bonding of the wafer to another wafer or a lid wafer, to encapsulate devices on either one of the wafers.
Transparent adhesives, such as SU-8 negative photoresist may be used as both an adhesive as well as a lithographic photoresist, for applications where light transmission through one or both wafers is required for proper functioning.
In each of these cases, it is important for the thickness of the liquid layer to be uniform. The thickness of the photoresist, for example, will affect the dimensional tolerances of the structures formed by the resist. The thickness of the adhesive layer may affect its light transmission properties, or the total thickness of the wafer stack, or the bonding strength between the wafers of the stack.
It is particularly problematic to form the uniform thin liquid film over wafers whose surface has topography formed therein. The movement of the liquid during application may be impeded by high lying areas, and collect in the low-lying areas such as trenches, vias, wells or depressions. The liquid collecting in these areas may interfere with the proper functioning of the device.